Motor speed control

ABSTRACT

A SYSTEM FOR ACCURATELY CONTROLLING THE SPEED OF A D.C. MOTOR WHICH IS POWERED BY THE GENERATOR OF A MOTORGENERATOR SET. THE FLUX IN THE GENERATOR IS CONTROLLED BY A GENERATOR FIELD SUPPLIED BY A SWITCHING BRIDGE AMPLIFIER. THE VOLTAGE INDUCED IN AN EXTRA GENERATOR FIELD IS INTEGRATED TO OBTAIN A MEASUREMENT OF THE CHANGE OF THE FLUX IN THE GENERATOR AND THIS MEASUREMENT IS USED TO CONTROL THE SWITCHING OF THE BRIDGE AMPLIFIER. THE CONTROL IS EFFECTED IN SUCH A WAY THAT THE RATE OF CHANGE OF FLUX IN THE GENERATOR IS MADE PROPORTIONAL TO THE ERROR BETWEEN THE PATTERN SPEED AND THE MEASURED SPEED OF THE D.C. MOTOR.   D R A W I N G

J. A. GINGRICH 3,706,017

MOTOR SPEED CONTROL Filed July 15. 1970 8 Sheets-Sheet 1 INVENTOR. JOHNA. GINGRICH g z /fidrl his ATTORNEYS QFK .w. 3 y El 3 mm NN WW 3 a Tum Wmcm Nw cw QQX Qw ml mi 1972 J. A. GINGRICH I 3,706,017

MOTOR SPEED CONTROL Filed July 15. 1970 8 Sheets-Sheet 2 T F/ 6. 3 60 RQ i i i a 5'5 6 5 I E E 1 5'4 i 1*? s: 9 e0 5 E i i l I (b) 5 F l6, I2536 544 542 FIG. /3 H6 INVENTOR.

JOHN A. GINGRICH @WQMZZLI, mw/F his ATTORNEYS Doc. 12, 1972 Filed July15. 1970 J. A. GINGRICH MOTOR SPEED CONTROL 8 Sheets-Sheet 5 I26 I08 I3484 90% 2 5 294 I22 L g I44 98 L) I38 M2 96 k g 'r '00 58 78 -wdv 80-lNVENTOR. JOHN A. GINGRICH fil mmdf his ATTORNEYS De. 12, 1972 J. A.GINGRICH MOTOR SPEED CONTROL Filed July 15. 1970 8 Sheets-Sheet 4 TieINVENTOR. JOHN A. GINGRICH @WZ LW QMQI f zymd/h/ his ATTORNEYS Dec. 12,1972 J. A. GINGRICH MOTOR SPEED CONTROL Filed July 15. 1970 FIG. /4

FIG. /5

Bridge Amplifier O Bridge Amplifier 8 Sheets-Sheet 8 570 F/G. l6-

INVENTOR. JOHN A. GINGRICH BY Kim/q 034W! his A TTOR/VEYS United StatesPatent O US. Cl. 318-146 18 Claims ABSTRACT OF THE DISCLOSURE A systemfor accurately controlling the speed of a DC. motor which is powered bythe generator of a motorgenerator set. The flux in the generator iscontrolled by a generator field supplied by a switching bridgeamplifier. The voltage induced in an extra generator field is integratedto obtain a measurement of the change of the flux in the generator andthis measurement is used to control the switching of the bridgeamplifier. The control is effected in such a way that the rate of changeof flux in the generator is made proportional to the error between thepattern speed and the measured speed of the DC. motor.

BACKGROUND OF THE INVENTION The present invention relates to vehicularspeed control systems Where a DC motor is driven by a variable voltageobtained from a motor-generator set. Such systems are particularlyuseful where the DC motor is alternately employed as a brake as well asa drive, such as occurs in a counterweighted elevator installation. Themotor is used to brake the elevator when the Weight of the descendingcounterweight exceeds the weight of the ascending car, :or when thedescending car is heavier than the ascending counterweight.

Precise control of the speed of a DC. motor in such a system is madedifiicult by the hysteresis and saturation of the generator magneticcircuit, and by changes of resistance due to temperature in the motorand generator fields and armatures. Were it not for those difiiculties,the speed would be exactly proportional to the current in the shuntfield of the generator, provided that a series field on the generatorwere adjusted to the correct value to compensate exactly for theresistance of the armatures and their connecting wires.

However, because of these difficulties, it is generally necessary to usesome form of feedback loop to get the accurate control of speed demandedby passenger elevator installations where fast, accurate and smoothperformance is required. Conventional feedback loops have been developedfor comparing the actual speed with a pattern speed, and for controllingthe generator field by some form of amplifier so that any error betweenpattern and actual speed is reduced to a small value. Generally, suchsystems are not entirely satisfactory for elevator installations.

One demanding requirement of an elevator installation which is notusually met by conventional feedback systerns, is that of bringing themotor speed down to zero, and of holding the motor stationary for abrief period until the brake has had time to fully apply. This must bedone over the full load range from no load on the car, to full load. Ifthe motor is still moving slightly, when the brake is applied, anobjectionable bump occurs.

Conventional feedback systems require a compromise with regard to theamount of damping; if the system is sufficiently damped to preventovershooting an oscillation, it may be too sluggish in forcing the speedto assume a new value dictated by the pattern; if the system is notdamped sufiiciently, the speed will overshoot and perhaps ICC oscillatebriefly when a new value of pattern occurs. Also, in order to reducehunting or overshooting of the speed, a steady state error may have tobe permitted. Systems which attempt to eliminate the steady state errorare generally prone to oscillatory ditficulties.

Much of the difficulty in conventional feedback loops is due to thedelay between a change in voltage applied to the generator field, andthe resulting change in speed. By the time the speed change is detected,it is too late to do anything further to the generator field, and thespeed will change beyond the amount desired. Part of this delay is dueto the inductance of the generator field; the flux in the generator, andhence the generator voltage changes exponentially when an abrupt changeof voltage occurs on the field. The remaining delay is causedprincipally by the mass of the moving parts of the system, including theinertia of the rotating parts such as the motor. In order to change thespeed, the generator voltage must differ from the motor armature voltageby a sufii cient amount to cause the necessary current to flow todevelop an accelerating or decelerating torque on the motor. Thearmature inductance also contributes to the delay, but its effect issmall in comparison with the two causes mentioned above.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to provide a feedback system for controlling the speed of agenerator-powered DC. motor in which the magnetic flux in the generatoris indirectly determined, and thus controlled with negligible delay, sothat the difiiculties due to delay between generator field voltage, andflux are eliminated.

A further object of this invention is to provide a feedback system forcontrolling the speed of a generatorpowered motor in which theresistance of the generator and motor armatures, which otherwise causesdelay and errors, is used to advantage to obtain smoothness andstability.

Another object of the invention is to provide a feedback system forcontrolling the speed of a generatorpowered motor which is capable ofcausing smooth, stepless acceleration of the motor at any prescribedrate.

A further object is to provide a feedback system for controlling thespeed of a generator-powered motor which is capable of bringing themotor speed down to zero, and holding it at zero for several seconds,without use of a friction brake, under all normal load conditions.

These objects, as well as other objects which will become apparent inthe discussion that follows, are achieved, according to the presentinvention, by providing a feedback system which causes the rate ofchange of flux in the generator to be proportional to the error betweenthe pattern speed and the measured speed of the motor.

In a preferred embodiment of the feedback system according to theinvention, the flux in the generator is controlled by a generator fieldsupplied by a switching bridge amplifier. The voltage induced in anextra generator field is integrated to obtain an indirect measurement ofthe net change in flux in the generator and this measurement is used tocontrol the switching of the bridge amplifier. In particular, theintegration is performed on the sum of the speed error and the voltageinduced in the extra generator field, and the bridge amplifier isswitched when the output of the integrator exceeds a predeterminedpositive or negative limit.

In a generator driven at constant speed, the generated voltage isdirectly proportional to the flux. On any field winding on thegenerator, the voltage induced in it is a measure of the rate of changeof the flux; this voltage is directly proportional to the number ofturns on the field, and to the time rate of change of the flux. Thus,integration of the voltage induced in a field of the generator indicatesthe net amount of change in flux, and this in turn, is proportional tothe net change in generated voltage. This generated voltage is directlymeasurable only if no armature current is flowing; otherwise, thevoltage drop in the armature causes the terminal voltage of thegenerator to dilfer from the internally generated voltage. Thus,integration of the voltage induced in a field on the generator canprovide an indirect measurement of any change in either flux orgenerated voltage.

In the preferred embodiment of the present invention an extra field isprovided on the generator for the purpose of measuring the flux changes.Frequently, such a field is available on generators; if not, the seriesfield may be used for this purpose since it is not required in thenormal way. The voltage induced in the series field is small, but stillsufficient. A higher voltage can be obtained by replacing the seriesfield with a winding of fine wire, with many more turns. Since thiswinding is not used to excite the field, and since the voltage inducedin it feeds into a high resistance, the current is very small and thusvery fine wire may be used, if space is limited in the generator.

In any feedback system, an amplifier is required. For the presentinvention, an amplifier is required to control the shunt field on thegenerator.- The amplifier need not supply the entire excitation,however; typically, it will supply about one-half of the totalexcitation, and the remaining half will be obtained by normal means,through contacts and resistors, or by self-excitation. The entireexcitation could, if desired, be entirely supplied by the amplifier.

For this invention, a switching bridge amplifier is particularlysuitable. Such an amplifier is described in the US. patent applicationSer. No.'656,758, filed July 28, 1967, and in the Canadian patentapplication No. 025,742, filed July 23, 1968, both in the name ofDemetre Iordanidis. This amplifier uses switching, rather than linearoperation, in order to minimize the heat dissipated in the transistors.Four main power transistors are used in a bridge circuit and only twoconditions are used: full output voltage of one polarity; or full outputvoltage of the opposite polarity. The average output voltage can bevaried by controlling the ratio of dwell times in the two conditions.Full reversible control of the output voltage is thus obtained. Thefrequency of switching can be considerably higher than line frequency,if desired; amplifiers employing silicon controlled rectifiers arenormally restricted to operation at line frequency.

The switching bridge amplifier is particularly suitable for thisinvention because it functions best when supplying an inductive load,such as a generator field, and also because it can induce an A.C.voltage, roughly a square wave, into the extra generator field. This AC.voltage is very convenient for integration between two fixed limits, andalso serves to sustain the oscillations of the system at a controlledfrequency determined by the integrator.

Basically, the signal obtained from the extra generator field isintegrated, and when the output of the integrator has reached one limit,such as for example, plus 7 volts, the bridge amplifier is reversed, andthe voltage induced in the extra field reverse s. This conditioncontinues until the output of the integrator has reached the otherlimit, for example, minus 7 volts; then the bridge amplifier switchesback to the original conditions.

This action repeats at a frequency of, typically, between 200 and 500hertz. Between reversals, the flux in the generator has changed by avery small, but definite amount. The flux tends to increase and decreaseby almost exactly the same amount each time, the accuracy of the amountbeing dependent upon the precision of the integrator. Thus, the averageflux remains substantially constant, but with a very small highfrequency ripple.

As mentioned, the accuracy of control depends upon the precision of theintegrator; with commercially available operational amplifiers, it ispossible to easily obtain typical drift rates as low as 2% of maximumflux per minute. More precision can be obtained by various methods, butis generally not required.

By applying a D.C. voltage, through a resistor, to the input of theintegrator so that it sums this voltage with the voltage induced in theextra generator field, it is possible to make the generator flux changeat a controlled rate instead of holding it constant.

In this mode of operation, the D.C. voltage adds to the induced voltagefor one polarity of the bridge amplifier, and subtracts for the oppositepolarity. Thus, the output of the integrator swings more rapidly in onedirection than the other, but still operates between the same twolimits. As a result, the flux in the generator is changed more in onedirection than the other so that after each cycle, it differs from theprevious cycle by a fixed amount.

This type of operation is also quite accurate, and the flux can becaused to increase very linearly with respect to time regardless of thehysteresis or saturation of the generator, regardless of the changes inresistance of the generator fields due to temperature, and regardless ofany variations in the line voltage.

Thus, the rate of change of flux, and hence the rate of change ofinternal generator voltage can be accurately controlled by an extrainput to the integrator. The delay between the changing of this inputand the commencement of the flux changing is at most one cycle; at 500Hz., one cycle takes only 2 milliseconds.

If the flux is caused to increase from zero at a controlled rate,smooth, stepless acceleration of the motor can be obtained. By makingthe flux rate of change proportional to the velocity error, accurate,stable, smooth control of speed is possible. These advantages will befurther explained in the description that follows, of the preferredembodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic drawing of aportion of the feedback circuit, according to the present invention, formeasuring and controlling the generator flux.

FIGS. 2a, 2b, 2c and 2d are diagrams showing various waveformsassociated with certain points of the circuit of FIG. 1.

FIGS. 3a and 3b are graphs illustrating the overall operation of thecircuit of FIG. 1.

FIG. 4 is a schematic drawing of the bridge amplifier shown as a blockin FIG. 1.

FIG. 5 is an equivalent electrical circuit for the generator and motorarmature connection.

FIGS. 6, 7 and 8 are diagrams showing waveforms which illustrate theperformance of the circuit of FIG. 5.

FIG. 9 is a schematic drawing of a more complete feedback systemaccording to the present invention, in which pattern speed is comparedwith the measured speed of the motor.

FIG. 10 is a schematic drawing of a circuit to produce a speed pattern.

FIG. 11 is a schematic drawing showing an alternative circuit forproducing a speed pattern.

FIGS. 12 and 13 are schematic drawings of auxiliary circuits whichimprove, the starting performance of the motor.

FIGS. 14 and 15 are schematic drawings showing modifications of thefeedback system according to the present invention.

FIG. 16 is a schematic drawing showing an alternative circuit formeasuring speed.

.5. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodimentsof the present invention will now be described with reference to FIGS.1-16 of the drawings.

In FIG. 1, there is shown a generator armature connected in opencircuit, so that the voltage e generated in the armature is the same asthe terminal voltage. It is assumed that the generator armature isrotated by a suitable prime mover, such as the induction motor of amotor-generator set, at a substantially constant speed.

An extra field 22 is shown, and the voltage e induced in it is appliedthrough resistor 24 to the inverting input 26 of an operationalamplifier 28. A further resistor 30, which is also connected to theinverting input 26 of the amplifier 28, has a voltage e applied to itfrom some other source, not shown in FIG. 1.

Two diodes 32 and 34 are used to protect the operational amplifier fromdamage due to excessive voltage, from either e or 6,. Normally, thevoltage between input 26 of the amplifier and ground is much lower thanthe forward drop of the diodes, and thus no appreciable current flowsthrough them.

A resistor 36, whose value should ideally be equal to the parallelresistance of resistors 24 and 30, connects the other input 38 of theamplifier to ground. This is in accordance with accepted practice foroperational amplifiers to minimize errors due to the zero offset currentof the device.

A capacitor 40, connected between the amplifier output 42 and theinverting input 26 causes the operational amplifier 28 to operate as anintegrator. The output voltage e should then be always proportional tothe time integral of the algebraic sum of input voltages e and e takinginto account the relative scale factors determined by resistors 24 and30. In a typical case, where the resistors 24 and take the values 150Kand 27K, respectively, e will have a greater efiect, by the ratio150/27, than e This is simply to allow e to control the integration froma low voltage supply, while e normally has higher values.

The output 42 of the amplifier 28 is applied through resistor 44 to thenon-inverting input 46 of a second operational amplifier 48. The output50 of the amplifier 48 is connected through resistor 52 to the input 46.A resistor 54, which ideally should be equal to the parallel resistanceof the resistors 44 and 52, connects the inverting input 56 of theamplifier 48 to ground.

In normal operation, the output voltage e of amplifier 48 is positivewhen voltage e is going from positive to negative; the actual voltage ofe is determined by the supply voltage to the operational amplifiers andthe load presented by the input 58 to the bridge amplifier. Forexplanation purposes, this voltage is assumed to be 10 volts.

Then, the 10 volts cause 1 ma. of current to flow through resistor 52into input 46 of the amplifier 48. When the voltage e exceeds about 6.8volts negative, the current through resistor 44 exceeds 1 ma., but inthe opposite direction, and thus the e voltage predominates over the avoltage and the output of amplifier 48 changes from plus 10 volts tominus 10 volts.

This change of a from plus 10 to minus 10 causes the output of thebridge amplifier 60 to reverse, so that now the output 62 will benegative, and the output 64 positive. This change reverses the voltageon the generator field 66, and also reverses the voltage induced in theextra field 22 so that voltage e now opposite in polarity, causes theintegrator comprised of the amplifier 28, resistors 24, 30 and 36 andcapacitor 40 to swing its output from negative to positive.

When it exceeds approximately 6.8 volts positive, the integrator outputvoltage e will again cause more than 1 ma. to flow through the resistor44, exceeding the 1 ma. flowing out through resistor 52, thus changingagain the output of the amplifier 48.

The purpose of operational amplifier 48, therefore, is to detect whenthe output voltage e from the integrator has reached either one of thetwo limits, and to initiate reversal of the bridge amplifier output wheneither limit is reached.

Waveforms are shown in FIG. 2 for voltages e a and e and for the fluxas, for the case where the voltage a, is Zero and where the flux 5 has asubstantial value. Just prior to time t the flux decreases toward thevalue rm, and induces a negative voltage on the output 68 of field 22,making e, negative (the output 70 of the field 22 is connected toground). The voltage e therefore rises and approaches the plus 6.8 voltlevel. In order to produce this polarity on the field 22, output 62 ofthe bridge amplifier 60 must be the same polarity as the output 68, andtherefore negative. This requires that the input 58 to the bridgeamplifier 60 and, thus, the output 50 of the amplifier 48, voltage e,,,be negative also.

At time t the voltage e reaches plus 6.8 volts. The operationalamplifier 48, therefore reverses the polarity of e from minus 10 to plus10 volts, as described previously. This causes the bridge amplifier toreverse its polarity, and thus the flux starts to increase at time t Atthe instant of reversal, the flux is at a value 5 Thereafter it risesalong a portion of an exponential curve for reasons which will bediscussed in further detail below. Since the flux has a substantialpositive value, the rate at which it rises is considerably slower thanthe rate at which it dropped prior to time t Because of the lower rateof change of flux, the voltage e induced in field 22 will be less. Theintegration will thus proceed at a slower rate, and the voltage e willtake a longer time to reach the minus 6.8 volt level.

At time t the flux reaches the value 5 and the voltage e reaches minus6.8 volts. This causes the output of the amplifier 48 to change fromplus 10 volts to minus 10 volts and reverses the polarity of the outputof the bridge amplifier so that the output 62 becomes negative. The fluxthen begins to decrease from the value It decreases exponentially, butat a higher rate in comparison with the rise between times 1 and tbecause it has a substantial positive value.

Thus, the voltage e, is now negative, and has a greater magnitude thanit had between times t and t This causes the integrator to change itsoutput e at a faster rate so that e rises from minus 6.8 volts to plus6.8 volts in a shorter period of time. At the time t;,, the flux hasagain reached the value 411.

This operation, as described above, will continue as long as inputvoltage 2 remains at zero. The flux will continue to rise and fallbetween two values ga and The difference between these two values isvery small in comparison with the average value of fiux, so that therising and falling of the flux represents a very small ripplesuperimposed upon the average value of the flux. The flux waveform inFIG. 2D has its 4: axis interrupted to allow the waveform to bedisplayed with the ripple suitably magnified.

It should be observed that the circuit of FIG. 1 is not capable ofdirectly measuring the flux; it is only capable of measuring the changein flux, in this case from to Over a period of minutes, any errors inthe integration could result in a gradual, but very small drift in theaverage value of the flux.

It should also be noted that, in general, if a constant voltage isapplied to the field of a generator, the current in the field willfollow approximately an exponential curve with respect to time. The fluxin the generator will therefore follow a similar curve, differingsomewhat due to hysteresis and saturation. Actually, as the magneticcircuit becomes more saturated, the inductance decreases and permits thecurrent to rise or fall more rapidly and partially compensate for thesaturation, making the flux follow more closely an exponential curve.

If the voltage applied to the field of a generator is changed from anegative to a positive value, for example, the flux will increaseexponentially from a negative to a positive value, the slope of theexponential curve decreasing as the flux becomes more positive. As theflux is changed, it requires more and more time to increase the flux bya specified amount.

Similarly, if the voltage applied to the field is changed from apositive to a negative value, there will again initially be a fastchange in the flux, with the rate of change decreasing as theexponential curve becomes more and more horizontal.

The above described behaviour explains why a negative change in the flux11: in the generator of FIG. 1 is more rapid than a change in thepositive direction, when the flux has a substantial positive value.

The circuit of FIG. 1 has been described for the case where inputvoltage e,, is zero, and where the flux has a substantial positivevalue. If the flux had a smaller value, the flux would rise and fallalong portions of exponential flux/time curves which lie closer to thetime axis, and the time between t and t and between t and t would bemore nearly equal. This, of course, results in a lower average DC.voltage applied to the generator field 66, and results in a lowercurrent through this field.

Similarly, if the average value of the flux is zero, the

' time between t and I is essentially equal to the time between t and tthe difference if any, is just enough to compensate for the hysteresisof the generator magnetic circuit.

If the frequency is high, the flux waveform consists of such smallportions of the exponential flux/time curves that the portions areessentially linear. Then, the waveforms of e, and e are comprisedessentially of straight lines as shown in FIGS. 2A and 2B.

Due to saturation of the magnetic circuit of the generator, the fluxdoes not exactly follow exponential curves when changing from one valueto another. However, the integrator still accurately measures the changein flux, and no errors occur due to saturation.

Another way of analyzing the circuit of FIG. 1 is to observe that theintegrator consisting of the amplifier 28, resistors 24, 30 and 36 andthe capacitor 40 cannot have a net D.C. input if the output is to remainbetween the limits imposed by the amplifier 48. Thus, with input e zero,the system requires an average value of zero for voltage e This in turnrequires that the average value of flux remain at a constant value, notnecessarily at zero.

' However, if the input voltage e has a value other than zero, theinduced voltage e must have a corresponding D.C. value of oppositepolarity to e and of higher magnitude by the ratio 150/27 of theresistors 24 and 30. Otherwise, the output of the integrator would gobeyond the limits imposed by the amplifier 48. Thus, the average valueof the flux must change at a rate proportional to input voltage e inorder to induce a suitable DC. voltage in the extra field 22.

The overall operation of the circuit of FIG. 1 thus permits the fluxand, hence, the generated voltage e to be accurately controlled by theinput voltage e This operation will now be analyzed in further detailwith reference to the exemplary waveforms in FIG. 3.

The overall performance of the circuit of FIG. 1 is illustrated by thewaveforms (a) and (b) in FIG. 3. Prior to time t,,, the flux is assumedto have a value which makes the generator voltage e positive, with avalue considerably less than rated voltage.

Between times t, and t a negative voltage is applied to input e, of thecircuit of FIG. 1. This causes the flux, and hence e to increase at aconstant rate.

Between times t and t the input voltage e is zero and thus voltage eremains constant.

Between times i and t-; a positive voltage, of greater magnitude thanbetween times t, and t is applied to input 8 e This causes e to decreasewith a proportionately greater slope than before.

Between times t and t the input e, is again zero, and thus the flux, andhence e remains constant.

Between times t and t the input a,, is changed linearly with respect totime to produce a parabolic shape to the waveform of e Here, the slopeof the e waveform gradually becomes more negative as e becomes morepositive.

Between times t, and r the voltage e is positive, but constant; thiscauses the voltage e to become increasingly negative at a constant rate.

Finally, at time t the input e is brought exponentially back towardzero. This produces an exponential rounding oil? of e to a negativevalue.

These waveforms in FIG. 3 illustrate how complete control of voltage ecan be accomplished by input e,,. The voltage e can be made to rise orfall at a constant rate, or parabolically, or exponentially by suitablycontrolling input e The overall circuit is basically, an integrator. Theoutput voltage 2 is proportional to the time integral of the inputvoltage e It is important to note, that in the more complete circuitsillustrated in later figures, the armature of the -gen erator isconnected electrically to the armature of a DC. motor, and the voltage ecannot be directly measured because of the internal voltage drop in thearmature due to armature current flowing through the armatureresistance. The terminal voltage, or the brush to brush voltage, differsfrom e by the IR drop.

Thus, the advantage of being able to control the voltage e becomes moreapparent.

One possible circuit for a bridge amplifier is illustrated in FIG. 4. Asuitable source of DC power is required to provide a positive voltage online 72 with respect to line "74 which is at ground potential. A voltageof 120 is a practical value consistent with available power transistors,and, for typical windings on generators. A bridge amplifier capable ofsupplying 2.5 amperes is generally sufficient to drive one-half of theshunt field on average sized generators. Power transistors capable ofhigher voltages or currents are available and could be used, if desired.

Also, line 76 requires a negative voltage; a voltage of minus 12. voltscould be suitable. This voltage would normally be obtained from the samesupply which provides plus and minus 12 volts (or some other suitablevoltage) for the operational amplifiers.

The input 58 to the bridge amplifier is assumed to be controlled by anoperational amplifier, such as the amplifier 48 shown in FIG. 1, so thatit always has either one of two voltages, for example, plus 10 and minus10 volts.

The output lines 62 and 64 are assumed to be connected to an inductiveload such as a generator field as shown in FIG. 1.

The circuit will be analyzed first for the case where input 58 is plus10 volts. Then, sufficient current flows through resistor 78 and intothe base of transistor 80 to turn it on. The emitter of transistor 80cannot assume a positive voltage, with respect to ground, of more thanabout 1 volt due to the diode action between the base and emitter oftransistor 82.

The turning on of transistor 80 causes a larger amount of current toflow from line 72 through resistor 84, through transistor 80 and intothe base of transistor 82. This turns transistor 82 on. The purpose oftransistor 80 is to amplify the current through resistor 78 sufiicientlyto turn on power transistor 82. A small portion of this current flowsthrough resistors 86, and 88 which function, at other times, to turntransistor 82 off.

The turning on of transistor 82 causes current to flow from line 72through resistor 90 and through Zener diode 92 and transistor 82 toground. The current through resistor 90 is suflicient to develop a fewvolts across Zener diode 92, which has a Zener voltage of 3.9 volts, toassist in turning off transistors 94 and 96.

The turning on of transistor 80 brings the base of transistor 94 downclose to ground potential, while the emitter of 96 is held a few voltspositive with respect to ground, as previously described. Thus,resistors 98 and 100 have current flowing upwardly through them. Thisdevelops voltage drops which assure that the bases of transistors 94 and96 are held negative with respect to their emitters. Thus, transistors94 and 96 are turned oflf.

The result, then, of turning on transistor 80 is to turn on transistor82, turn off transistors 94 and 96. This causes the output 64 of thebridge amplifier to be connected to ground through transistor 82.

In addition, input 58, being about 10 volts positive, causes current toflow through resistors 102 and 104 to ground; this current holds thebase of transistor 106 slightly positive so as to turn transistor 106off.

This permits current to flow from line 72 through resistor 108, throughZener diode 110 and through resistor 112 to the negative supply line 76.Zener diode 110 has a Zener voltage of volts, and the current throughthe resistor 112 which has a value suitable to develop a drop of about3.5 volts. Therefore, the base of transistor 114 is held at about 3.5volts negative with respect to ground (the 12 volts of line 76 less the3.5 volt drop in the resistor 112 and the 5 volt drop in the Zener diode110). Transistor 114 is therefore turned otf.

With transistor 114 turned off, the base of transistor 116 is held at anegative value by the voltage divider effect of resistors 118 and 120,since current flows from ground through these resistors to the negativesupply line 76.

Also, the turning off of transistor 114 causes the base of transistor122 to rise to a high positive voltage. The transistor 122 operates asan emitter follower to raise the base of the transistor 124 to almost ashigh a voltage. Transistor 124 also operates as an emitter follower topull up the output 62 toward the level of line 72.

If no current flows through the load, the output 62 is held at a highpositive voltage. If current flows from line 72 through the choke 126and resistor 128, and then through the transistor 124 and Zener diode130, and out line 62 to the load, a voltage drop occurs across theresistor 128 which assures sufiicient current through the resistor 132to turn on transistor 122, and thus assuring that the transistor 124 isturned on sufliciently.

Thus, the result of turning off the transistor 114 is the connection ofoutput 62 to the positive supply line 72 through elements 126, 128, 124and 130 and the turning off of the transistor 116.

Thus, the application of plus volts to the input '58 of the bridgeamplifier causes the output 64 to be grounded, and the output 62 to beraised to a positive value close to the voltage of line 72.

Similarly, it can be shown that application of minus 10 volts to input58 causes the output 62 to be grounded and the output 64 to be raised toa positive value close to the voltage of line 72.

When input 58 is at minus 10 volts, a voltage divider consisting ofresistors 134 and 78 causes the base of transistor 80 to be about 2.5volts negative with respect to ground. Transistor 80 is thus turned off.The turning oif of the transistor 80 produces a result similar to thatof turning 011 the transistor 1'14, described above, except that now itis transistor 82 which turns off, and transistors 94 and 96 which turnon.

Also, when the input 58 is at minus 10 volts, current flows from groundthrough the emitter-base of the transistor 106 and through the resistor102 to the input 58. This turns the transistor 106 on, and its collectorthen rises up to almost ground potential. This enables current to flowfrom line 72 through the resistor 108 into the base of transistor 114 toturn it on. The voltage drop across the Zener diode 110 is then lessthan its Zener voltage, so that it passes only negligible current.

The turning on of the transistor 114 is similar to the 10 turning on ofthe transistor described previously, except that now it is thetransistor 116 which turns on, and the transistors 1'22 and 124 whichturn Oh.

The basic action of the bridge amplifier has now been described. Theinput 58, by being positive or negative, causes power transistors toconnect the output lines 62 and 64 to the supply line 72 and 74, in onepolarity or the other. A small drop in voltage is accepted, throughZener diodes 92 and and also through resistor 128, and choke 126, inorder to obtain the necessary currents or voltages to turn the varioustransistors on or off. In spite of this, most of the line voltagebetween lines 72 and 74 can be applied across the load.

The load current does not necessarily flow through the power transistors96 and 116 or 124 and 82, however. Assume that while input 58 isnegative, a substantial amount of current is flowing from line 72through elements 126, 128, 96, 92, the load, and 116. Then, when input58 becomes positive, transistors 96 and 116 turn off, and transistors124 and 82 turn on.

The current, however, cannot reverse suddenly due to the inductance ofthe load, and a voltage is induced in the load which is suflicient tokeep the current flowing through the path from line 74 through diode136, the load, and through diodes .138 and to the line 72. This currenttends to charge a capacitor 142, which has sufficient capacity toprevent the line 72 from raising its voltage appreciably during thebrief time when this inductive current flow continues.

If the bridge amplifier remained in this condition long enough, withinput 58 positive, the current would eventually reverse and it wouldthen flow through elements 124, 130 and 82. Normally, however, thebridge amplifier remains in one condition for only a brief time.

The purpose of the diodes 136, 138, 140, 144 and 146 is to provide apath for such inductive flow of current so that excessive voltagescannot appear across the transistors. The purpose of choke 126 is toprevent dangerous spikes of current if, momentarily during switching,there occurs a path through elements 96, 92 and 82 or through elements124, 130 and 116.

The circuit shown in FIG. 4 is only an example of a switching bridgeamplifier. Any suitable circuit which can reverse the polarity of aninductive load may be used in this invention.

FIG. 5 shows the equivalent electrical circuit for the usual connectionbetween the generator armature 20 and the armature 148 of the motorwhose speed is to be controlled. Normally, the two armatures arepermanently connected together by heavy wiring, possibly through thecoil of an overload relay.

In FIG. 5 the total armature resistance, consisting of the resistancesof both armatures plus the connecting wires, has been lumped into oneresistor 150 with a value of R, ohms. The two armatures, then, areassumed to have zero resistance for analysis purposes. This well knownmethod of circuit analysis allows the internally generated voltage e ofthe generator and the internal voltage e of the motor armature to beisolated for explanatory purposes from the armature voltage drop R,,i,,.The current in the armature loop is 11,.

It is assumed that the field 152 of the motor has a constant currentflowing through it from a suitable power supply. Then, the speed of themotor is proportional to the voltage e It is also assumed that the fieldof the generator is supplied by a bridge amplifier and circuit such asshown in FIG. 1.

This equivalent circuit of FIG. 5 is to be used in the analysis below inconnection with the graphs of FIGS. 6, 7 and 8.

FIG. 6 illustrates the behaviour of the voltage e when 2 rises at asteady rate from zero to a plateau, remains there for a short period,and then descends to zero at the same rate. It is assumed that the loadon the motor does not have an unbalanced condition such as acounterweight heavier or lighter than an elevator car. An elevatorsystem, with the load on the car adjusted so that the total car weightequals the counterweight, would be a suitable load for the curves ofFIG. 6.

For the major portion of the period where e is rising at a constantrate, voltage c is also rising at the same rate, but separated from itin time by the amount required to make the voltage difference between eand e sufficient to cause a flow of current i of the correct value toaccelerate the mass. This separation between e and e is automatic andnon-oscillatory. Obviously, if e lags too far behind, it causes a highercurrent and this accelerates the motor at a higher rate. If e approachese too closely, the current decreases and the acceleration ratedecreases. Thus, the e voltage is forced to rise at the same rate as thee voltage.

Since the speed of the motor is proportional to e the acceleration ofthe load, such as an elevator car, is constant over most of the periodwhere 2 is increasing linear- 1y. It is only during the initial periodthat the acceleration of the load is not constant, but the effect ishighly desirable since it results in a gradual build up to the fullacceleration rate.

Thus, the resistance R of the armature loop automatically smooths outthe initial starting of the load so that the acceleration builds upgradually, rather than abruptly. Consequently, the bump or jerk (i.e.the second derivative of velocity with respect to time) is notexcessive. Experience indicates that this efiect is generally sufthanallow it to stop abruptly. Also, for practical reasons,

it is best to not follow the slope shown in FIG. 6 since it is diflicultto predict in advance the exact instant at which e must stop rising inorder that the speed, when finally rounded off, be exactly the desiredtop speed. A more satisfactory technique, which will be discussed inconnection with FIG. 8, is to make the slope of the re curveproportional to the error between pattern speed and the actual speed,but never greater than a predetermined value.

The remainder of FIG. 6 shows how the voltage e is reduced to zero whenthe voltage e decreases linearly to zero. In this case, e,, is less thane by an amount equal to R, z,. since the same slope is used. If e wereto decrease at a more rapid rate, the current i would have to be greaterin order to decelerate the load at a more rapid rate, and the separationbetween the two curves would be greater.

FIG. 7 shows a similar situation to that shown in FIG. 6; in this case,however, the motor has an unbalanced load such as would occur with anascending elevator having a greater than balanced load in the car. Herethe separation between e and e is greater during acceleration becausethe current i must have an additional component, in addition tothatrequired to accelerate the mass, in order to counteract the unbalance.Similarly, the separation is less during deceleration for the samereason; now the unbalance is assisting in the reduction of speed.

In order to increase e to the same maximum value as in FIG. 6, e mustrise to a higher level so that, at constant speed, a suitable R f dropexists to compensate the unbalance. Similarly, when the speed is reducedagain to zero, the e voltage must not decrease as far as zero. The netresult is that the entire e curve must be raised above the level it hadin FIG. 6.

If the e voltage can be raised to an appropriate value,

depending on the unbalance, just prior to releasing the 12. brake, the ecurve of FIG. 7 can be made identical to the e curve of FIG. 6. However,this is difiicult to do. If the e voltage starts out from zero in thesame manner as in FIG. 6, the e voltage, and thus the speed, willreverse briefly due to the unbalance after the brake re-' leases,assuming that the brake releases at the same time that e starts rising.This efiect is the familiar roll-back which frequently occurs when afully loaded elevator first starts an up run. A circuit will bedescribed in connection with FIGS. 12 and 13 for detecting such aroll-back, and forcing e to rise at a considerably higher rate while themotor is rotating backwards to thereby reduce the rollback to a verysmall distance.

FIG. 8 shows the performance of a system in which the rate of change offlux, and thus the rate of change of e is made proportional to thevelocity error, but never greater than a predetermined value. A balancedload condition, as was the case in FIG. 6, is assumed. The velocityerror is the difference between a pattern voltage e and the speed asindicated by the voltage e In a practical system, where e cannot bemeasured directly, some other method or device for measuring speed wouldbe used, such as a tachometer generator. The pattern voltage producedmight be of considerably lower magnittude than e and possibly ofopposite polarity.

For explanatory purposes, the pattern voltage e is assumed to riseabruptly at time t to a value calling for full speed. The error betweene and e is then quite high, more than enough to demand the maximum rateofincrease for voltage e Thus, between time t and time 1 tll lleGsyztemoperates in the same manner as the system of However, at time the erroris now much smaller; in particular, it is just the amount required toproduce the maximum rate of increase on voltage a After time r the slopeof the e curve is proportional to the difference between the voltages eand e As time progresses, this error decreases, and thus the slope ofthe curve decreases. The result is an exponential rounding off to thetop speed. This curve exhibits a more gentle and conventional roundingofi to top speed than does the curve shown in FIG. 6.

The remaining portion of FIG. 8 shows what happens when the patternvoltage e drops suddently to a some What lower level. Immediately, theerror between e and e calls for an equivalent rate of decrease in thevoltage e but as the speed decreases, the difference between e and edecreases and thus the slope of the e voltage decreases. The result is asmooth reduction in speed from the previous level to the new onedemanded by the pattern voltage e A typical slowdown for an elevatormight consist of a series of steps of reduction in a pattern voltage,such as the single one illustrated in FIG. 8.

FIG. 9 shows a circuit to accomplish the results shown in FIG. 8. Thevoltage applied to pattern input 154 dictates the speed; this voltage isproportional to the e voltage of FIG. 8, and its polarity determines thedirection of rotation of the motor.

The circuit of FIG. 9 is suitable for operating an elevator. The hoistmotor for the elevator has its armature 148 connected electrically tothe generator armature 20. The field 152 of the hoist motor is excitedwith substantially constant current of a suitable magnitude while themotor is running, or preparing to run; the current in the motor fieldmay be lowered to a lesser value when the car has stopped in order toreduce the heat dissipated in the motor as is normally done in elevatorinstallations.

A tachometer generator is'used to provide a voltage proportional to thespeed of the motor. A DC. tachometer is desirable since its polarity isan indication of the direction of rotation of the motor. This tachometeris mechanically coupled to the motor by any suitable means such asfriction drive, direct drive or by belts or gears. The excitation forthe field of, the tachometer generator 13 may be derived from permanentmagnets or from a suitably wound field supplied with substantiallyconstant current.

The armature 156 of the tachometer generator is connected between groundand resistor 158 so that a current proportional to speed flows throughresistor 158 into or out of the inverting input 160 of an operationalamplifier 162. The inverting input normally departs from groundpotential by such a very small voltage that, for practical purposes, itcan be assumed to be at ground potential. It is assumed that thetachometer armature is so connected that upward motion of the elevatorcauses a positive voltage to be applied to the resistor 158 and downwardmotion causes a negative voltage to be applied to the resistor 158.

This polarity of the tachometer requires that the pattern input 154 benegative for up and positive for down. The value of resistor 164 can bechosen to suit the voltage level available from the pattern source. In atypical case, the tachometer voltage may be /5 volt for each foot perminute of elevator speed. Then, a car speed of 500 f.p.m. would produce100 volts. If the resistance of resistor 164 were made 68K ohms and theresistance of resistor 158 made 680K ohms, it would require voltspositive on the input 154 to dictate a speed of 500 f.p.m. in the downdirection, and similarly 10 volts negative on input 154 to dictate 500f.p.m. up.

Resistor 166 connects the non-inverting input 168 of amplifier 162 toground; resistor 166 is assumed to have a value equal to the equivalentof resistors 158, 164 and 170, in parallel. Protective diodes 172 and174 are also used, as for amplifier 28 in FIG. 1, to protect theoperational amplifier 162.

A clamping circuit consisting of diodes 176, 178, 180 and 18:2 and Zenerdiode 184 is connected between ground and the output 186 to theamplifier 162. Two matched Zener diodes could be used in series betweenthe output 186 and ground, as an alternative. The purpose of thisclamping is to prevent the voltage at the output 186 from exceeding apredetermined value which is somewhat lower than the supply voltage tothe operational amplifiers. In FIG. 9 it is assumed that plus and minus'12 volts is used to supply the operationalamplifiers, and that theZener diode 184 has a Zener voltage of about 9 volts so that, wth theadditional voltage drops in the diodes, the voltage at the output 186 ofthe amplifier 162 is prevented from swinging more than about 10 voltspositive or negative with respect to ground.

A voltage divider, consisting of resistors 188, 190 and 192, isconnected as a load on the output of the amplifier 162, and the slidingtap 194 on the resistor 190 is connected through resistor 170 back tothe inverting input 160 of the amplifier 162. This arrangement causesthe voltage on the output 186 of the amplifier to assume a valueproportional to the difference between the currents in resistors 1 64and 158, and thus proportional to the error between the pattern speedintroduced at the input 154 and the speed as measured by the tachometergenerator.

This use of an operational amplifier is well known and is sometimesreferred to as a summing amplifier. Any unbalance in the currentsthrough the resistors 170, 164 and 158 swings the voltage at theinverting input 160 in such a direction as to make the output 186 swingin the opposite direction, highly amplified, until the new value ofcurrent through the resistor 170 .balances the difference between thecurrents in the resistors 164 and 158.

If the slider 194 is moved toward the resistor 188, more current flowsthrough the resistor 170, for a given voltage at the output 186 and thusa larger error, between pattern speed and measured speed, is required toproduce a given voltage at the output 186. Similarly, if slider 194 ismoved toward the resistor 192, a smaller error is required to produce agiven voltage at the output 186.

The voltage obtained from the output 186 of the amplifier 162 isbasically applied, through a voltage divider, to a circuit almostidentical with the circuit of FIG. 1. Thus, the velocity errordetermines the rate of change of flux in the generator, and operationsimilar to that illustrated in FIG. 8 is obtained. Generally, the fullclamped voltage of plus or minus 10 volts on the output 186 produces thefull acceleration value which can be adjusted to a suitable value by aslider 196- on a resistor 198.

The effect of the slider 194 is to determine how much velocity error isrequired to produce the fully clamped swing of the voltage at the output186 and hence, to produce the full acceleration value. Thus, if slider194 is set near the resistor 188 (upper end), a large velocity error isrequired to produce full acceleration, and therefore, as illustrated inFIG. 8-, the speed starts to round off, as at time i when the measuredvelocity is very much lower than the pattern velocity. This results in avery gentle rounding oil? to full speed. Alternatively, if the slider194 is set near the resistor 192 (lower end), the measured velocity canrise to a value much closer to the pattern speed before the time isreached, and then the round ofi to top speed is more abrupt.

Thus, potentiometer /194 can be adjusted for very smooth, but perhapstoo sluggish response, or for very quick, but perhaps too bumpyresponse, and in between these two extremes a setting can be found whichis suitable.

In addition to this, diodes 200 and 202 and contacts 204 and 206 can heused to obtain, in effect, different settings of the slider 194 for thetwo parts of a typical run: first, the acceleration up to top speed;second, the slowing down and stopping. Relay 204 is assumed to beenergized to cause the car to run in the up direction; it picks up atthe beginning of the run, possibly just before the brake is energized,and drops out when the car reaches the final stopping point, usuallyabout /2 inch from floor level. It does not pick up during a relevellingoperation. Similarly, relay 206 is assumed to be energized for a downrun, but not when the car relevels down. Such relays are commonly usedin elevator systems, and circuits suitable for operating such relays areshown in the Canadian Pat. No. 774,755 and in the corresponding US. Pat.No. 3,407,905.

When an up trip is in progress, the contact 204 is closed and thus diode200 is connected across a portion of the resistor 188; the slider 208determines how much of the resistor 188' is shunted by the diode. Duringthe acceleration to top speed, in the up direction, the polarity of thevoltage at the output 186 is positive and thus the diode 200 has forwardcurrent through it. This diode therefore bypasses a portion of theresistor 188 and has the same effect as moving slider 194 toward theresistor 188, causing a more gentle round off to top speed. However,when the car is slowing down, but still travelling in the up direction,the voltage at the output 186 is negative, and the diode can pass nocurrent.

Similarly, when a down trip is in progress, the contact 206 is closed toconnect the diode 202 across the same portion of the resistor 188.During the acceleration to full speed in the down direction, the voltageat the output 186 is negative, and forward current flows through thediode 202 to bypass part of resistor 188. During the slowing down, inthe down direction, the voltage at the output 186 is positive and thediode 202 passes no current.

Thus, the slider 208 may be adjusted to obtain a considerably smootherround off to full speed, without affecting the performance duringslowdown. This adjustment is particularly desirable on trips from onefloor to an adjacent one where the motor does not reach full speed. Inthis case, the pattern voltage at the input 154 may be decreasingrapidly at the instant that the rising motor speed causes the voltage atthe output .186 to begin changing from its clamped value in one polaritytowards its clamped value in the opposite polarity, in order to changethe motor from acceleration to deceleration. Slider 208 can be used tocause this process to begin earlier, and thus provide a more gentlechange from acceleration to deceleration.

A further voltage divider, consisting of resistors 210*, 198 and 212, isconnected between the output 186 and ground. Slider 196 on potentiometer198 is then used to create a voltage e,, which is applied to theresistor 30in a circuit closely similar to that of FIG. 1. Slider 196can be adjusted to give a suitable value of acceleration. Moving ittoward the resistor 210 gives a greater rate of increase of generatorflux, and thus a greater rate of increase for voltage e and thus ahigher value of acceleration.

A further circuit consisting of diodes 214 and 216, and contacts 218 and220 can be provided to allow for a larger value of acceleration(actually, deceleration) when the motor is slowing down than the valueof acceleration when the motor is accelerating. Such a circuit is notrequired if a single step of slowdown is used, as might be used oninstallations where the speed is relatively low, such as'200 f.p.m. Inthis case, at a fixed distance from a floor where a stop is to be made,the pattern would abruptly drop, from a voltage corresponding to fullspeed to a voltage corresponding to landing speed, and this lattervoltage would remain until the car reached the final stopping point,possibly about /2 inch from floor level.

For higher speeds, however, it is preferable to have a larger number ofsteps of slowdown or even a continuously varying pattern based on carposition. In this case, the speed of the elevator is controlled by thepattern voltage during slowdown, and the system should be capable of asomewhat higher value of acceleration (actually, deceleration) duringthe slowdown so that there will be no tendency for the clamping of thevoltage at the output 186 to limit the performance, and thereby cause anovershoot, if the pattern should decrease slightly more rapidly than themotor speed is allowed to decrease as determined by the setting of theslider 196.

When an up run is in progress, the contact 218 is open; consequently thediode 214 is disconnected and the diode 216 is in parallel with theresistor 210. During the acceleration to full speed, the voltage at theoutput 186 is positive,.and the diode 216 passes no current. During theslowing down of the motor, the voltage at the output 186 is negative,and the diode 216 shorts out the resistor 210 to increase the currentthrough the resistors 198 and 212. This increases the voltage on theslider 196 and is equivalent to moving the slider 196 toward theresistor 210.

Similarly, when a down run is in progress, the contact 220 is open andthus the diode 216 is disconnected and the diode 214 is in parallel withthe resistor 210. During the acceleration to full speed in the downdirection, the voltage at the output 186 is negative and. the diode 214passes no current. During the slowing down, the voltage at the output186 is positive and the diode 214 shorts out the resistor 210 toincrease the voltage on the slider 196.

During a relevelling operation, the contacts 204 and 206 are open, andthe contacts 218 and 220 are closed. Thus, the resistor 210 is shortedout by the diode 214 for one polarity and by the diode 216 for the otherpolarity while the resistor 188 is not shorted with either polarity.This results in the fastest response because the shorting of theresistor 210 gives the higher value of generator flux change, for agiven voltage on output 186 and the resistor 188 being not shorted givesthe higher value to the output 186 for a given error.

The remaining portion of FIG. 9 contains a circuit similar to FIG. 1,however, with several differences. Additional input resistors 222 and224 are connected to the input 26 of the operational amplifier 28. Thepurpose of the resistor 222 is to allow for a voltage e to be applied togive a rapid change of generator flux if the motor moves in the wrongdirection when starting. A circuit for doing this will be described inconnection with FIG. 12.

A further feedback circuit is shown in FIG. 9 to obtain what is known assuicide action when the motor has stopped. This action is to hold thegenerator voltage at a low value so that excessive currents cannotcirculate in the motor and generator armatures.

A voltage divider consisting of the resistors 226, 228 and 230 isprovided to obtain a voltage across the resistor 228 which isapproximately of the generator terminal voltage. The connection betweenthe resistors 226 and 228 is grounded. The connection between theresistors 228 and 230 is connected through the contact 232 to theresistor 224 at the input 26 of operational amplifier 28.

Relay 232 is arranged to be energized whenever the motor is running, andfor about 1 second after the brake is de-energized when the car stops.Thus, the contact 232 is open for running, and closes when suicideaction is res quired. Such a contact is used in most elevator systems,

and a circuit suitable for operating such a relay is shown in FIG. 2 ofCanadian Pat. No. 774,755 or the corresponding U.S. Pat. No. 3,407,905.

When the car is running, the contact 232 is open and the resistor 224has no effect on the amplifier 28. When the motor has been slowed to alanding speed and is to be stopped, the pattern voltage at input 154 ischanged to zero and, at the same time, the brake is de-energized.Suflicient inductive delay is normally built into the brake to preventit from applying before the motor has come to a stop. Contact 232 mustremain open until the brake has applied. The system initially sees zeropattern voltage with a tachometer voltage indicating continuing motionof the motor. This causes the system to change the level of flux in thegenerator in such a direction as to bring the car to a stop. When it hascome to a Stop, the system holds the flux at that level in order to holdthe car stationary while the brake applies. The system illustrated inFIG. 9 is particularly effective in bringing the motor to a smooth stop.However, if the flux remained at this level, which could besignificantly different than zero if the load on the car wereunbalanced, high currents could continue to circulate in the motor andgenerator armatures.

The closing of the contact 232 causes the application of a voltagethrough the resistor 224 to operational amplifier 28 of such a polarityas to cause the flux to be reduced to a very low value. If, for example,the right side of the generator armature 20, which has the polarity dotadjacent to it, is positive, it is necessary to apply a more negativevoltage on the output 62 of the bridge amplifier 60, tending to make theright side of armature 20 also more negative. This suicide action bringsthe generator armature voltage down to near zero, and holds it there.During this time, it is assumed that the voltage at the input 154 iszero and that the tachometer voltage is also zero and that, therefore,the e,, voltage applied to the resistor 30'is zero.

A further difierence between the circuit of FIG. 9 and the circuit ofFIG. 1 is the addition of the resistor 234 and the contact 236. Therelay for the contact 236 is assumed to be equivalent to a brakecontactor; such a relay is required on any traction elevatorinstallation, and circuits for energizing such a relay are well known.Thus, when the motor is running, the contact 236 is closed and theresistor 234 is shorted out so that the feedback circuit from the field22 has the same resistance as in FIG. 1. When a stop is called for, thecontact 236 opens and introduces the extra resistance 234. This requireshigher voltages to be induced in field 22 and, hence, the flux mustchange more rapidly. This circuit modification can cause the motor to bebrought more rapidly to a final stop, without affecting the rest of therun.

The circuit of FIG. 9 is particularly suitable for an installation wherethe input pattern voltage has many small 17 steps of reduction duringthe slowdown of the motor, with each reduction occurring at a particulardistance between the car and the floor it is Stopping at, oralternatively where the pattern is stepless and controlled by thedistance between the car and the floor it is stopping at.

FIG. 10 shows a circuit for obtaining a large number of small steps ofpattern reduction. It is based on the system described in Canadian Pat.No. 774,755 and in the US. Pat. No. 3,407,905 where an electromagnet inthe hoistway, called a position magnet, is energized at a floor wherethe car is to stop. The approach of the car to that floor results in theclosing of a sequence of various proximity contacts mounted in avertical column on the car while each proximity contact is in theoperating range of the electromagnet. The operating range depends uponthe dimensions of the magnet, but it is assumed that the dimensions aresuch that once a proximity contact has closed, it remains closed forabout 12 inches of car travel. The circuit of FIG. 10 is based on thepositioning of the various proximity contacts in such a way that, duringa slowdown, there is always enough overlap so that at least one isclosed at all times. This means that adjacent contacts must not bespaced apart vertically by more than 12 inches.

This arrangement of proximity contacts allows deletion of speed relaysV3, V4, V5, V6, V7 and V8 shown in FIG. 3 of the abovementioned patents.The proximity contacts are not used to control the dropping out of speedrelays, but instead are used to directly produce a pattern voltage. Aseries of equal resistors has a constant current passed through them,and the voltage across these resistors is the pattern. The proximitycontacts short out more and more of these resistors as the carapproaches the floor, to give a diminishing voltage for the pattern.Since these resistors are mounted on the car near the proximitycontacts, the number of wires required between the car and the controlequipment in the machine room is re-- duced, and is independent of thenumber of steps of slowdown.

In FIG. 10, a constant current source, consisting of the resistors 238,240 and 242, transistor 244, Zener diode 246 and diode 248, can beenergized from a positive supply line 250 when contact 252 closes. Therelay for the contact 252 is assumed to be a down relay or contactorwhich is energized at the same time as the relay for contact 236 whichwas described in connection with FIG. 9. Thus contact 252 closeswhenever the motor is required to move in the down direction.

If the Zener diode 246 has a Zener voltage of volts, the resistor 238can be adjusted to carry the constant current over a range from wellbelow, to well above 40 ma. The purpose of this adjustment is to enablethis constant current source to be adjusted to exactly match anotherconstant current source, used for the up direction, which is notadjustable but which has a nominal value of 40 ma.

The output of this constant current source is applied to the wire 254,and this constant current then flows through the resistor 256, the diode258, and then through a long series arrangement of equally valuedresistors 260, 262, 264, 266 268, 270, 272 and 274 and then throughadjustable resistors 276, 278, 280 and 282 to ground. A portion of the4-0 ma. current also flows through resistors 284 and 286 to ground, butsince the resistance of this path is much higher than the other path,the majority of the current flows through the former path, and anegligible amount through the latter.

The purpose of resistors 284 and 286 is to provide an adjustable portionof the voltage developed on line wire 288 as a pattern voltage. Thesliding tap 290 on resistor 284 is intended to connect to input 154 inFIG. 9. The voltage developed in wire 288 is proportional to theresistance between wire 288 and ground, since a constant current flowsthrough this resistance.

If all proximity contacts are open, a voltage of typically 28 voltswould be created on wire 288; this voltage is dependent on the settingsof resistors 276, 278, 280 and 282. Such a condition exists if theproximity contacts have not yet entered an energized position magnet.

As the car approaches a floor where a stop is to be made in the downdirection, the proximity contacts close in the order 292, 294, 296, 298,300, 302 304, 306, 308, 310, 312. The closing of the proximity contact292 causes the voltage on wire 254 to decrease by about 8.8 volts (4-0ma. times 220 ohms, the value of resistor 256), and thus it assumes thesame voltage as wire 288 which does not change. The purpose of thecontact 292 and the wire 254 will be apparent later.

The closing of the proximity contact 294 allows the 40 ma. constantcurrent to bypass the diode 258, which has a forward voltage drop ofapproximately 0.8 volt. The voltage on wire 288 is thus reduced byapproximately 0.8 volt.

The subsequent closing of the proximity contact 296 bypasses the diode258 and the resistor 260. This causes a further reduction of 0.88 volt(40 ma. times 22 ohms, the value of the resistor 260) in the voltage onwire 288. The proximity contact 294. can now be opened without affectingthe voltage on wire 288.

Similarly, the subsequent closing of the proximity contact 298 causes anadditional reduction of 0.88 volt (40 ma. times 22 ohms, the value ofthe resistor 262) and also permits the contact 296 to open withouteffect.

A similar reduction of 0.88 volt on the line 288 occurs for each newclosing of a proximity contact. If several proximity contacts remainclosed at once, there is no change in operation. All that is required isthat each proximity contact remain closed at least until the next one inline is closed.

Thus, as the car approaches a floor where a stop is to be made, thepassage of the proximity contacts on the car through the energizedposition magnet(s) at the floor results in a large number of equalreductions in the voltage on wire 288, and, hence, in the patternvoltage at the input 154 of the circuit of FIG. 9.

After the proximity contact 312 has closed, the contacts 314, 316 and318 operate in a manner described in the previously referred to patents.The closing of the contact 316 bypasses the resistor 276, the closing ofthe contact 314 bypasses the resistor 278 and the closing of the contact318 bypasses the resistor 280 leaving only the resistor 282 which can beadjusted to obtain a suitable pattern voltage for landing speed.Finally, when the car is about /2 inch from floor level the energizationof the relay for contact 320 causes the relay for contact 252 to bede-energized so that the 40 ma. constant current source is disconnectedand the pattern voltage at input 154 becomes zero to call for zerospeed.

For the up direction, the contact 322 on an up relay or contactor causesenergization, from a negative supply line 324, of a further constantcurrent source consisting of resistors 326 and 328, diode 330, Zenerdiode 332 and transistor 334. Energization of this constant currentsource causes the constant current to flow from ground through resistors282, 280, 278, 276, 336, 338, 340, 342 344, 346, 348 and 350, diode 352,and resistor 256. This develops the opposite polarity on the wire 288and hence on the pattern voltage at the output 290.

The reduction in pattern voltage as a car approaches a floor in the updirection is similar to the previously described operation for the downdirection. Proximity contacts 354, 356, 358, 360, 362, 364 366, 368,370, 372 and 374 close in sequence; then contacts 320, 314 and 318 closein sequence and finally the energization of the relay for contact 316causes the relay for contact 322 to be de-energized so that the contact322 opens and removes power from the constant current source to causethe pattern voltage at the output 290 to become zero.

Only the last four steps are made adjustable to allow for varying thefinal approach performance. For the macontacts. For example, if theproximity contacts 312 and 310 are separated by 3.0 inches, and thecontacts 310 and 308 are separated by 3.3 inches, then the contacts 308and 306 should be separated by 3.6 inches, and each suceeding dimensionshould increase by 0.3 inch. This results in a spacing of 10.5 inchesbetween the contacts 296 and 294, and a total distance of 175.5 inchesbetween the contacts 312 and 294, if there are 17 proximity contactsbetween 302 and 304. Such an arrangement might be suitable for anelevator installation with a speed of 600 f.p.m.

The purpose of the diodes 258and 352 is to direct the constantcurrentthrough the correct string of resistors dependent upon thedirection of the current flow (which is dependent upon the direction oftravel). Although a single string of resistors could have been used forboth directions, the arrangement used in FIG. 'has the advantage ofsimplified wiring, since the proximity contacts 356 and 294, forexample, may be arranged in separate columns of proximity contacts or,if in the same column, will be a considerabledistance apart.

Thus, it can be seen that the arrangement of FIG. 10 provides a slowdownpattern consisting of many small reductions in voltage, and that eachadditional step, if more are required, is obtainable at a cost of twomore proximity contacts and two more /2 watt resistors. This system doesnot provide a pattern for accelerating the car; the pattern voltagejumps immediately to a high value, possibly representing full speed, atthe instant that notching occurs as the car is about to leave a floor.However, the circuit of FIG. 9 does not require any accelerationpattern; it is capable of providing a smooth acceleration.

In the system described in the previously mentioned patents, CanadianPat. No. 774,755, or the US. Pat. No.

3,407,905, a relay CP was used to determine when to notch. At fullspeed, notching occurs when the proximity contact 354 or 292 closes.During acceleration to full speed, notching may occur when a criticalposition is reached; If notching does not occur when this criticalposition is reached, the pattern created by the proximity contacts willcommence to reduce the acceleration, and will create a pattern'whichwill cause the car to slow downand stop at the floor at which theposition magnet is energized. If no stop is desired at this floor,notching ahead to another fioor must occur at or before the criticalposition.

If the circuit of FIG. 10 is used to create a pattern voltage, relaysV3, V4, V5, V6, V7 and V8 in FIG. 3 of the previously mentioned patentsare no longer available to operate the relay CP. vTherefore, in FIG. 10,an additional circuit is provided to operate a relay 376, preferably asmall relay such as a reed relay, which can be operated directly from anoperational amplifier. A contact of the relay 376, such as contact 378can then be used to energize the relay CP.

When no proximity contacts are closed, the constant current must flowthrough the resistor 256, and thus the wire 254 has a voltage greaterthan wire 288 by about 8.8

' volts as described previously. A circuit consisting of diarmature 156shown in FIG. 10 is the same tachometer as in FIG. 9, and that theresistors 396, and 398 have the same ratio of resistances as have theresistors 164 and 158 in FIG. 9.

If so, the current through the resistor 396 will generally exceed thecurrent through the resistor 398 while the extra 3.8 volts is present onwire 390 with the car running at full speed. However, if the proximitycontact 354 or 292 closes, the voltage on wire 254 will become the sameas the voltage on wire 288, thereby lowering the voltage on wire 390 by8.8 volts to make it 5 volts lower than wire 288. The current in theresistor 398 will then exceed the current in the resistor 396.

The operational amplifier 400 has resistors 396 and 398 connected to itsinverting input 402, and a resistor 404, the resistance of which equalsthe equivalent of 396 and 398 in parallel, connects the non-invertinginput 406 to ground. No feedback resistor is used, so that the gain ofthe operational amplifier is a maximum. Thus the output 408 generallyassumes either a high positive value or a high negative one, dependingupon which of resistors 396 and 398has the greater current. Protectivediodes 410 and 412 prevent harmful voltages from appearing at the inputof the amplifier 400.

For the up direction, the contact 414 is closed and one side of the coilof the relay 376 is connected to the same positive supply line 416 whichsupplies the amplifier 400 and the other operational amplifiers. Also,the negative I constant current source is energized by the contact 322.

these may be ganged so that the two are moved in unison by a singleshaft. Also, it is assumed that the tachometer Therefore, when all theproximity contacts are open and the current through the resistor 396exceeds the current through the resistor 398 (which current has theopposite direction since the tachometer voltage is positive with respectto ground) the output 408 of the amplifier 400 is positive and close tothe positive voltage on line 416 so that the voltage applied to the coilof the relay 376 is insufficient to close the contact 378.

However, when proximity contact 354 closes, the current through theresistor 396 decreases to less than the current through the resistor398, and the output 408 of the amplifier 400 becomes negative. The coil376 then has a sufiicient voltage to cause the contact 378 to close.

Similarly, for the downdirection, the contact 418 closes so that oneside of the coil of the relay 376 is connected to the negative supplyline 76 which supplies the ope tional' amplifiers, including theamplifier 400, and the contact 252 energizes the other constant currentsource. When all the proximity contacts are open, the currents in theresistors 396 and 398 are such as to cause the output 408 of theamplifier 400 to become negative so-that the relay 376 will not beenergized. When the proximity contact 292 closes, the relay 376 isenergized because the output 408 of the amplifier 400 becomes positive.

Thus, when the car is operating at fnllspeed, each closure of aproximity contact 354 or 292 causes the con-.

tact 378 to close, thus energizing the relay GP to either causenotching, if no stop is to be made, or to signal the commencement ofslowdown if a stop is to be made.

This circuit also operates during acceleration to full speed. In thiscase, any proximity contact which is closed by the position magnet forthe next floor ahead causes the voltages on wires 254 and 288 to beessentially the same because the drain of current through the resistors394 and 420 to ground is so small that, when flowing through theresistor 256 and some of the 260 or 350 string of resistors which aremuch smaller, an insignificant voltage drop occurs. Thus, theoperational amplifier 400 is continuously checking the rising tachometervoltage against the decreasing pattern voltage. When these two voltagesapproach within about 5 volts, or whatever drop occurs in the Zenerdiode 388 and its associated diodes, the relay 376 is energized by theamplifier 400 to signal the need for notching if no stop is to be made.

The purpose of the Zener diode 388 is to anticipate the approach of thedescending pattern voltage to the rising 21 tachometer voltagesufiiciently early to assure that notching occurs before any reductionin acceleration occurs, so that the acceleration can proceed on to fullspeed without slackening. If the notching occurred too late, theacceleration would begin to reduce as if a stop were required at thisfloor.

A small capacitor 422 can be used to render the amplifier 400insensitive to random spikes of voltage on the wire 390 or from thetachometer armature 156.

FIG. 11 shows an alternative circuit for creating a pattern voltage.This circuit is particularly suitable for an installation whose selectorsystem cannot provide a large number of small steps of reduction inpattern. Many conventional selectors are capable of providing only asmall number of coarse steps of slowdown.

In the circuit of FIG. 11 the main shunt field on the generator has beenwound in two parts, 66 and 424, so that the bridge amplifier need onlysupply about one-half of the total excitation. The extra field 22 isalso required as before, for the measurement of flux change in thegenerator.

In FIG. 11 the generator field 424 is energizable from the positivesupply line 72 through a resistor 426, contacts 428 and 430 or 432 and434, through the contact 436 and through the resistor 438'. For eitherdirection, up or down, the current through this field flows through theresistor 438 in a direction such that the slider 440 develops a positivevoltage, with respect to ground, which is proportional to the currentthrough the field 424.

Adjustable taps 442, 444, 446, 448 and 450 are provided on the resistor426 to allow for adjustment of the various levels of current, andcontacts 452, 454, 456 and 458 are provided in the well known way tocontrol the current through the field 424. During acceleration, relays452, 454, 456 and 458 are energized in sequence, typically throughtimers, and during slowdown these relays drop out in the sequence 458,456, 454 and 452 at predetermined distances from a floor. For thecircuit of FIG. 11, it is best to have no timing on these relays.

The voltage developed on the slider 440 is then used as an approximatepattern voltage. Smoothing is required because of the coarseness of thesteps and inversion is needed to enable reversal of the polarity of thefinal smoothed pattern. Although capacitive smoothing could be used, amuch superior form of smoothing will now be described.

In FIG. 11, a negative smoothed pattern is developed on the output 460of the operational amplifier 462, and the positive smoothed pattern isdeveloped on the output 464 of the operational amplifier 466.

The principal inputs to the amplifier 467 are through the resistors 468,from slider 440, and 470' from the negative smoothed pattern. If thesetwo inputs are exactly in agreement (that is, with the negative smoothedpattern equal to R47'OV R468 times the voltage on the slider 440) but ofopposite polarity, the output 472 of the amplifier 467 will have avoltage of zero. The ratio R470/R468 is the ratio of resistances of theresistors 470 and 468. If these two inputs to the amplifier 467 are notin agreement, the output 472 becomes positive or negative with respectto ground; it becomes positive if the negative smoothed pattern is toogreat, and negative if the negative smoothed pattern is too small.

If the effect of the resistors 474 and 476 is ignored, the output 472 ofthe amplifier 467 will swing to the fully allowable amount, asdetermined by the Zener diode 478 and the diodes 480, 482, 484 and 486for the slightest error between the two inputs.

If for example, the negative smoothed pattern voltage is too small, theoutput 472 of the amplifier 467 will swing to the full negative clampedvalue, which will be assumed to be volts. This causes an integrator,consisting of the resistors 488 and 490, a capacitor 492 and theoperational amplifier 466, to integrate a constant negative input whichthus causes its output to become more positive at a con- 22 stant rate.This output is inverted by the operational amplifier 462 so that thenegative smoothed pattern becomes more negative at a constant rate. Thiscontinues until the negative smoothed pattern is in agreement with thevoltage on the slider 440.

With no feedback resistor for the amplifier 467, an oscillation of smallamplitude and high frequency can occur at the output 472. Therefore, afeedback resistor 476, connected between the output 472 and theinverting input 494, is used to prevent such oscillation. This resistoralso causes the negative smoothed pattern to round off slightly as itnears equivalence with the voltage on the slider 440.

Thus, the purpose of the operational amplifier 467 is to detect anyerrors between the unsmoothed pattern on the slider 440 and the negativesmoothed pattern, so that the latter can be brought into agreement withthe former at an adjustable, but constant rate.

A voltage divider consisting of the resistors 496, 498 and 500 is usedto adjust the rate of change of the smoothed pattern voltage and also toallow for a higher rate for decreasing the smoothed pattern than therate for increasing it. When the output 472 of the amplifier 467 ispositive, the diode 502 bypasses the resistor 496 to give a greatervoltage on the slider 504; when the output of the amplifier 467 isnegative, the diode 502- has no effect, and a lesser voltage exists onthe slider 504. Therefore, the integrator has two different slopes: alesser one for increasing its output, and a greater one for decreasingits output.

The purpose of this difference in operation is to allow adjustment ofthe slider 504 to give a suitable rate of climb of the smoothed pattern,in order to get suitable value of acceleration to full speed, withoutthe danger that the smoothed pattern will decrease to zero so slowlythat it cannot follow the steps of slowdown on the unsmoothed pattern.Ideally, during slowdown, the smoothed pattern should come briefly intoagreement with each step of the unsmoothed pattern before a new stepoccurs. This arrangement gives excellent smoothing, with negligibledelay, whereas the conventional capacitive smoothing has an appreciabledelay if made equally smooth.

The purpose of the resistors 474 and 506, and the diode 508, is to allowfor an adjustable amount of round off of the smoothed pattern when itapproaches full speed, with no effect during slowdown. While thesmoothed pattern is increasing, the voltage on the output 472 isnegative, and current fiows through the resistor 506 and diode 508. Thenegative voltage applied through the resistor 474 is added to thenegative voltage applied to the resistor 470, so that the amplifier 467begins earlier to bring its output to zero, and as it does, the voltageapplied to resistor 474 decreases. This results in exponential roundingoil of the smoothed pattern to its final value. The potentiometer 506thus has an effect similar to that of the potentiometer 188 in FIG. 9.This feature provides for a smooth operation on a single floor run,where the smoothed pattern might otherwise change too abruptly fromincreasing to decreasing.

The only purpose of the operational amplifier 462, and resistors 510,512 and 514, is to invert the output 464 of the integrator; suchinversion is required to provide the correct polarity to feed back intothe amplifier 467, and also to enable contacts 516 or 518 to connect theappropriate polarity to the resistor 30.

In FIG. 11, the tachometer armature 156 is connected diiferently than inFIG. 9, but it is still assumed that a positive voltage is applied tothe resistor 158 for the up direction and a negative voltage for thedown direction of rotation of the motor. The difference here is that theerror between pattern voltage, as applied to resistor 30,,and thetachometer voltage is not obtained from a separate operational amplifiersuch as the amplifier 162 in FIG. 9; instead, the difierence betweenthese two voltages is automatically obtained by applying them both tothe opera- 23 tional amplifier 28. The result is the same as if theerror in velocity had been applied to the amplifier 28 as in FIG. 9.

The remainder of FIG. 11 is generally similar to FIG. 9 and to FIG. 1.As in FIG. 9, a resistor 222 is shown to allow for application of avoltage e, to prevent rollback as will be explained below in connectionwith FIG. 13.

The voltage induced in the extra field 22, is not applied directly toresistor 234, as in FIG. 9. Instead, resistors 520 and 522 permit theattenuation of this voltage via the slider 524. This slider thusprovides an adjustment similar to that of the slider 194 in FIG. 9.

An additional suicide circuit consisting of contacts 526 and 528 isprovided in addition to the one using contact 232. This is aconventional suicide circuit, and can be used to assure suicide actionif the regulator is disconnected. The motor can be operated atapproximately half speed, if the regulator is disconnected, because ofthe conventional circuit which supplies the field 424.

The purpose of the resistor 530 is to provide a path to ground for theresistor 30 when the contacts 516 and 518 are both open. I

In FIG. 12, the voltage from the tachometer armature 156 is appliedthrough a resistor 532 to the inverting input of an operationalamplifier 534. A resistor 536 is connected as a feedback resistor fromthe output to the inverting input of the amplifier 534. The value of theresistor 536 is made high so that the tachometer voltage will be highlyamplified. Thus, a very low speed of the motor will swing the output ofthe amplifier 534 to its maximum positive or negative value. For higherspeeds, diodes 538 and 540 bypass the current from the resistor 532, toprotect the operational amplifier.

If a run in the up direction is commencing, a contact 542 closes, and aslong as the motor rotates only in the up direction, its polarity ispositive with respect to ground. The output of amplifier 534 istherefore negative, and no current flows through the diode 544. However,if the motor moves in the down direction, as it might with a full loadin the car, the negative voltage from the tachometer causes the outputof the amplifier 534 to become positive, and forward current then flowsthrough the diode 544, contact 542 and the resistor 546.

Similarly, if the contact 548 is closed and the motor moves in the updirection, the output of the amplifier 534 becomes negative, and currentwill flow from ground through the resistor 546, contact 548 and thediode 550.

The slider 552 on the resistor 546 is intended to be connected to theresistor 222 in FIG. 9. Thus, if the car moves in the wrong direction asthe car is starting, a voltage e, is applied to the resistor 222 in sucha direction as to make the generator flux rise at a much more rapid rateas long as such backward motion is detected. This tends to reducesignificantly the rollback shown in FIG. 7.

FIG. 13 shows a circuit which has the same purpose as the circuit inFIG. 12 but which is suitable for use in conjunction with the circuit ofFIG. 11. A voltage of opposite polarity is required in the circuit ofFIG. 11 for the resistor 222, because, although the tachometer polarityis the same as in FIG. 9, it is connected into a dilferent part of thecircuit.

Therefore, an additional operational amplifier 554 is used to invert theoutput of the amplifier 534, and the diodes 544 and 550 are connectedwith a polarity opposite to that shown in FIG. 12.

FIG. 14 shows several possible modifications to the circuit of FIG. 9.First, the main shunt field on the generator is wound in two parts, sothat the bridge amplifier is only required to supply a portion of thetotal excitation via field 66and the remainder is supplied by theselfexcited field 424 through an adjustable resistor 556.

Contacts 558 and 560 can be arranged to open when the elevator doors areopened to limit the speed it the bridge amplifier fails at that time,and a further contact 24 562 can be arranged to open after the car hashad time to come to a normal stop, or after the doors are fully open, toinsert a resistance 564 in series with the field 66 for added safety.

FIG. 15 shows an additional modification to the circuit of FIG. 11.Here, additional contacts of the speed relays 452-458 are used to insertmore and more portions of the resistor 564 in series with the field 66to reduce the overshoot, particularly at terminal floors, if the bridgeamplifier fails.

Although a DC. tachometer generator has been shown in many of thepreceding figures, it is possible to measure the motor speed by othermeans. If the motor has interpoles, it is convenient to use the motorarmature terminal voltage as a measure of speed, and to use the voltagedrop in the interpole as a measure of the internal IR drop of thearmature. The reason for using the interpole is that its temperaturetends to be similar to the temperature of the armature. The temperatureof an external resistor would not be the same.

In FIG. 16, the connection between the motor armature 148 and the motorinterpole 566 is grounded through a protective fuse 568. The other motorlead is used to measure the motor speed by using the resistor 570' toconnect to the input of a suitable operational amplifier, such as theamplifier 162 in FIG. 9 or 28 in FIG. 11.

A potentiometer 572, connected across the interpole, allows a suitableproportion of the interpole voltage to be applied, through a contact 574and a resistor 576 to the same operational amplifier as the voltageacross the resistor 570. The combined effect of the currents in theresistors 570 and 576 is equivalent to the current from a tachometergenerator.

The opening of the contact 574 results automatically in suicide action,eliminating the need for the contact 232 and the associated circuits, inthe circuits of FIG. 9 or 11.

Although the present invention has been described by referring toseveral possible circuit configurations and embodiments, numerousvariations and modifications falling within the scope and spirit of thisinvention, are possible. Accordingly, it is intended that the presentinvention be limited only by the following claims.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. Apparatus for controlling the speed of a DC. motor to follow apattern speed, said apparatus comprising, in combination:

(a) a generator, connected to power said DC. motor,

having a first generator field;

(b) means for producing a signal representative of the error between thepattern speed and the measured speed of said DC. motor;

(c) means for measuring the rate of change of flux in said generator;and

(d) control means, connected to said signal producing means, to saidmeasuring means and to said first generator field, for applying avoltage to said first generator field to cause the rate of change offlux in said generator to be approximately proportional to said errorsignal.

2. The apparatus defined in claim 1, wherein said control means includesa switching bridge amplifier for applying a prescribed voltage ofalternating polarity to said first generator field.

3. The apparatus defined in claim 1, wherein said measuring meansincludes a second generator field, arranged in said generator andconnected to said control means, for producing a voltage proportional tothe rate of change of flux in said generator.

4. The apparatus defined in claim 3, wherein said control means includesmeans for summing at least a portion of said error signal and saidvoltage produced by said second generator field, and producing a signalrepresentative of this sum.

